A substantial fraction of those infected with the hepatitis C virus (HCV) have immune complexes in the serum. These may be asymptomatic, or they may cause symptoms of mixed cryoglobulinemia. We have found that many HCV patients have a significant increase in the frequency of circulating B cells. These cells were not increased as a result of activation, since HCV patients had an increase in B cells of a resting, naive phenotype. Almost all HCV patients studied had increased numbers of B cells that resemble transitional cells recently released from the bone marrow. We have also found that approximately half of the patients studied have immune complexes directly associated with peripheral blood B cells, suggesting that some immune complexes may be overlooked in screens of serum. Immune complexes were found independently of B cell frequency. The long-term goal of this proposal is to understand why immune complexes are produced in HCV patients, and how these complexes affect viral binding and tropism. We will isolate immune complexes from the serum as well as from the B cell surface. Using quantitative PCR methods, we will determine what fraction of the virus in the blood is actually associated with immune complexes in the serum and on cells. Using a novel pseudotype assay for viral attachment and entry, we will test the hypothesis that immune complexes facilitate viral entry into target cells or the transfer of virus from one cell type to another. In addition, we will evaluate the role of chemokines found in the blood during HCV infection, in the accumulation of B cells with a native or T1/T2 transitional phenotype in HCV patients. A mouse model will be developed to study the effects of overproduction of specific chemokines in the liver, on the frequency and phenotype of circulating B cells. This may improve our understanding of the genesis of lymphoid malignancy in HCV patients. These studies will improve our understanding of the origins of cryoglobulinemia, a significant extra hepatic manifestation of HCV infection. By characterizing the roles of immune complexes in cell entry, these studies may also lead to a better understanding of HCV pathogenesis and improved design of therapeutic and preventive vaccines.